Macrorheology of cystic fibrosis, chronic obstructive pulmonary disease &amp; normal sputum

David J Serisier,Mary P Carroll,Simon A Young,Janis K Shute

doi:10.1186/1465-9921-10-63

Abstract

BackgroundPrior microrheologic assessments of selected, microlitre plugs of cystic fibrosis (CF) sputum suggest no intrinsic rheologic abnormality. However, such analyses may not be representative of CF sputum as a whole. We therefore reassessed this question using whole sputum macrorheology. Additionally, we wished to further explore the relationships between sputum rheology, inflammation and infection.MethodsDynamic oscillatory macrorheometry was performed on whole expectorated sputum from stable adults with CF (n = 18) and COPD (n = 12) and induced sputum from normal controls (n = 7). Concomitant sputum inflammatory mediator levels were measured in CF and COPD samples. Sputum collected from CF subjects (n = 6) at commencement and completion of intravenous antibiotic therapy for an infective exacerbation was also assessed.ResultsCF sputum neutrophil elastase activity (NE) was significantly related to degree of sputum purulence (p = 0.049) and correlated significantly with measures of sputum viscoelasticity (r = 0.696, p = 0.008 for storage modulus G' at 9 Hz). There were significant differences in viscoelasticity between subject groups when samples were compared irrespective of appearance/degree of sputum purulence. However, the macrorheology of mucoid CF sputum did not differ from normal sputum (eg median (range) G' at 9 Hz 2.25 (0.79, 3.26) vs 2.04 (1.4,4.6) Pa, p = 1). In contrast, mucoid COPD samples demonstrated significantly greater viscoelasticity (G' at 9 Hz 4.5 (2.4, 23) Pa) than sputum from both CF (p = 0.048) & normal subjects (p = 0.009). Antibiotic therapy during exacerbations was associated with significant reductions in CF sputum viscoelasticity, with mean (SD) G' at 9 Hz decreasing from 28.5 (11.5) Pa at commencement to 6.4 (4.6) Pa on day 7 (p = 0.01).ConclusionThe macrorheologic properties of whole, mucoid CF sputum are not different from normal, confirming the results of prior microrheologic studies. Instead, CF sputum viscoelasticity is related to secondary infection, decreases with intravenous antibiotic therapy and correlates with inflammation. In contrast, COPD sputum demonstrates inherently greater viscoelasticity, providing a novel target for potential therapeutic interventions.

Highlights

Prior microrheologic assessments of selected, microlitre plugs of cystic fibrosis (CF) sputum suggest no intrinsic rheologic abnormality
The exact pathophysiology linking cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction with CF pulmonary disease remains unclear, the popular airway surface liquid (ASL) volume theory [1,2] predicts that dehydrated, thickened respiratory secretions are integral to mucociliary stasis and the subsequent development of CF airway pathology
There are a number of potential limitations of some previous investigations, including the use of high shear rates that may disrupt the mucus gel network of respiratory mucus [7,8], the comparison of CF sputum with non-human 'normal' mucus [6] and the analysis of microlitre quantities of selected sputum plugs that raise the possibility of sample bias given the heterogeneity of CF sputum [6]

Summary

Introduction

Prior microrheologic assessments of selected, microlitre plugs of cystic fibrosis (CF) sputum suggest no intrinsic rheologic abnormality. Such analyses may not be representative of CF sputum as a whole. Simple visual inspection of CF sputum reveals its obvious, marked heterogeneity This visual variability is confirmed in sputum measures including inflammatory markers [10] and rheology, with work from our and other groups showing large variations in viscoelasticity when analysing separate aliquots from the same sputum samples (coefficients of variation of up to 150% for aliquots from the same samples) [11,12]. This extreme variability must create concern over the validity of analyses that use small selected sputum portions

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Discussion

Conclusion